276 Part III. Chapter 3. 
group with simple topography San Clemente, with extremely young topography 
in generally hard rocks; San Nicolas, with young topography in soft rocks; 
San Pedro Hill with modified mature topography, now presenting, on the 
whole, the forms of adolescence, developed in moderately soft rocks; San 
Miguel, with modified mature topography, though belonging, according to its 
general character, with the more rugged islands.. To the group with rugged 
topographic forms belong Santa Catalina, Santa Cruz and the larger part of 
Santa Rosa. It is probable, that all of the islands originated through crustal 
deformation, for the most part, as tilted blocks’’). 
The botany of the Californian islands is of interest on account of the 
variations from mainland forms, largely due to isolation and the effect 
of insular climate. The list of species found on the islands contains about 
512 species. Twenty six species of the list have not yet been found on the 
mainland, though all but three are near relatives of coast forms and twelve 
of these twenty six plants have been found on the islands off the coast of 
Lower California, leaving only fourteen species which are probably peculiar to 
the islands. 
There are a number of species and genera peculiar to the islands and 
these peculiarities in part with other characteristics may be ascribed t0 the 
survival of an old flora on the islands, while that of the mainland has DR 
more or less modified by the intrusion of new forms from other regions since 
the separation of the islands from the continent proper. Lyonothamnus, CO 
sisting of two very distinct species, has no very near relative in any © ae 
‚part of the world. Crossosoma another genus of shrubs has one species IN 
digenous to several islands with none on the immediately neighboring mal“ 
land, though a second small and insignificant member of the genus occurs ON 
the verge of the interior deserts. Four species of Zavatera are scattered UP 
and down the archipelago, while not a single species is indigenous to kr 
American continents, all the generic allies being of the Mediterranean region, 
with the exception of three or four, which are confined to remote an“ 
oceanic islands. Another negative point of divergence between the insular 
and mainland floras is the almost or total absence from the islands of such 
prevalent mainland genera as Ribes, Lupinus, Astragalus, Potentilla, Horkelia, etc 
Equally remarkable is the fact, that certain trees, shrubs and herbaceous p!# 
long known as extremely rare, or quite local, on the mainland, such a8 # = 
Torreyana, Malacothrix incana and Leptosyne gigantea, occur ON the islan 
in the most luxuriant abundance. Such plants also as Potentilla anser " 
Cressa cretica, faumea carnosa and two species of Salicornia confined to 5% 
marshes occur on San Miguel Island and not on the others, because none © 
the others have as much as a square rod of that kind of ground. Az ” 
"Fornide 
1) SmitH, W. S. TANGIER: A topographic Study of the Islands of southern Califo 
Bulletin Department Geology University of California II: 179 
a ee 
