I'd like to know if in this excerpt from The Great Gatsby, chapter II, the word "mass" means "crowd" and if "solemn" means "dreary", "lugubrious," like in "His eyes, dimmed a little by many paintless days under sun and rain, brood on over the solemn dumping ground" :
Upstairs in the solemn echoing drive she let four taxicabs drive away before she selected a new one, lavender-colored with grey upholstery, and in this we slid out from the mass of the station into the glowing sunshine.
These are hard words to define in this context. I believe that the clue is the word "upstairs." Evidently, they have to go upstairs to get to the street level and then are still inside the station. The taxis must be pulling into an enclosed driveway to pick up passengers. I imagine that this enclosure has a high vaulted ceiling resembling that of the interior of a cathedral and that the resemblance to a cathedral with a similar ceiling is what makes the place feel solemn. It would also have a cathedral-like quietness because of being enclosed and shut off from the street. If an author used the word "solemn" to describe the feeling of being inside an impressive religious edifice I doubt if you would have any trouble with the adjective, since you are undoubtedly familiar with the feeling. The cathedrals are designed with the intention of making you feel solemn, or very serious.
As far as the word "mass" is concerned, I don't believe it has anything to do with people but that it is an impression of a huge, rambling structure with many entrances and exits, many different kinds of rooms, and probably several different levels. In other words, it is "massive," and when they finally exit from this sprawling architectural monstrosity, it is a relief to get out from under it and into the sunshine, as if they are being relieved of a great weight.
I don't believe that "solemn dumping ground" means exactly "dreary" or "lugubrious" but that it is so gray, cheerless, and silent, so much like a vast cemetery, that it evokes solemn thoughts and feelings not unlike the feelings evoked by a cathedral. The word "solemn" as applied to the dumping ground also suggests to me a feeling of finality, as if anything that ends up here has reached its ultimate end.
My dictionary defines "lugubrious" as "Mournful to an exaggerated or ludicrous degree." I have always thought of the word as suggesting a certain pleasure in painful feelings, somewhat like that operatic aria “Vesta la giubba” in Pagliacci by Leoncavallo. Someone who is lugubrious is so mournful that it almost becomes funny.