The Secret Doctrine is built on a device. Blavatsky presents a series of Stanzas, said to be translated from an archaic work called the Book of Dzyan, and then comments on them at very great length. The stanzas themselves are short and oracular; the commentary is where the content is, and it runs to well over a thousand pages across two volumes.
The first volume, Cosmogenesis, treats the emanation of the universe: the periodic manifestation and reabsorption of worlds, the sevenfold division of everything, and a cosmology explicitly offered as an alternative to both Genesis and contemporary physics. The second, Anthropogenesis, treats the emergence of humanity through a sequence of root races, and it is the part of the work with the worst subsequent history, because its racial schema was taken up in the twentieth century by people Blavatsky would not have recognised and put to uses the atlas records rather than passes over.
The commentary form matters for how the book is read. Because the stanzas are unverifiable and the commentary is vast, the argument can always retreat from the text to the exposition, and there is no external check available on either.
No manuscript of the Book of Dzyan has ever been produced, and Blavatsky said it was inaccessible. The atlas records the stanzas as her composition unless and until a source is found, which is the position the academic study of the subject takes.