Crosby.] 494 [April 7, 
Next in chronological order comes the quartz-porphyry and fel- 
stone series. ‘This occurs mainly in the central part of the Colony, 
covers anarea of several thousand square miles, and rests at all 
points upon the granite. These rocks, which present many varieties 
in both texture and composition, are said to occur sometimes in the 
form of dykes; but, after a careful study of the descriptions and sec- 
tions, I am strongly persuaded that they are chiefly indigenous. 
Many passages might be quoted in which the bedded nature of the 
porphyries is either asserted or distinctly implied, although Messrs. 
Brown and Sawkins, like most English geologists, belong to the plu- 
tonic school. Associated with this petrosiliceous group and forming a 
part of it are various hornblendic, slaty and other more nondescript 
rocks, all distinctly bedded. But, whatever the origin of the group, 
I think there can be but little doubt that it corresponds in age to 
the great petrosilex series of the Andes and southern Brazil and 
may be provisionally referred to the Huronian system. 
_ The third crystalline group, composed of gneisses and schists, rests, 
in different places, upon both the preceding groups, and probably 
embraces the newest stratified crystallines inthe Colony. The gneiss 
predominates and is described as sometimes granitic or syenitic, but 
it is usually highly micaceous and distinctly stratified or schistose. 
The bedding is often excessively contorted, and passages into true 
mica schist are common. ‘The feldspar is white, and large crystals are 
sometimes porphyritically interspersed. Garnet is the principal ac- 
cessory constituent, and veins of coarse granite holding tourmaline, 
garnet and other minerals are mentione@ as frequently occurring. 
The schists are still more various. Mica schist prevails, but chlo- 
rite and hornblende schists, steatite and quartzite are also included. 
Garnets are abundant in the mica schist, and the coarse endogenous 
granite is found equally in this rock and the gneiss. 
The gneiss and schist series extends into French Guiana; and, ac- 
cording to Jannetaz, parts of the mica schist are rich in staurolite as 
well as garnet, while tourmaline, rutile and zircon also occur. The 
Montalban characteristics of this formation are very marked, and no 
argument is required to show that it agrees closely with the upper 
part of the older crystalline series of Brazil, which I have already 
referred to the Montalban series. 
Jannetaz has pointed out the fact that there is a very exact corre- 
spondence between the crystallines of Brazil and French Guiana; 
and besides the points of resemblance already considered I may men- 
