THE DIKES 405 



allel to the general line of folding in this region. There are some ex- 

 ceptions, hut that is the usual course. This is to be expected, since 

 cracks or lines of weakness would naturally develop parallel to the 

 anticlinal axes, and would hence be the lines along which the dike 

 magmas would be injected. It will also have been noticed that the 

 major axis of the igneous area is also approximately parallel to this. 



The dike rocks. — The dikes are composed of " nepheline-tinguaites " 

 and leucite-tinguaites, basic nepheline-porphyries, fourchites, and mon- 

 chiquites. They have been so thoroughly described by Williams that 

 the reader is referred to his work for details. I would merely remark 

 that the rock which he calls " nepheline-tinguaite " comes more prop- 

 erly under the head of tinguaite-porphyry, as used by Pirsson,* ortho- 

 clase phenocrysts being abundant, though the groundmass is quite 

 black and aphanitic. Some of his leucite-tinguaites also contain no 

 leucite, but are very typical tinguaites proper. The nepheline-porphy- 

 ries and the larger masses of monchiquitic rocks, as well as the largest 

 dikes *of tinguaite-porphyry, are found in the hornstone area along the 

 Ridge. Leucite-tinguaite seems to occur only in close connection with 

 the leucite-porphyry as irregular schlieren-like masses rather than in 

 dike form, especially toward the borders. They may best be regarded 

 as merely facies of this or of the foyaite or shonkinitic syenite. 



Chemical composition. — The analyses here given illustrate the chemical 

 characters of these dikes. It will be seen that the tinguaites resemble 

 the three syenites closely. The tinguaite-porphyry especially is almost 

 identical with the foyaite, and one of the leucite-tinguaites also resem- 

 bles it, except that Fe 2 3 and K 2 are considerably higher. The " neph- 

 eline-felsite " seems to represent an intermediate type not found among 

 the abyssal rocks. The nepheline-porphyry, fourchite, and monchiquite, 

 on the other hand, resemble rather the ijolite. This is especially true 

 of the nepheline-porphyry, as was noticed by Williams. f 



The fourchites and monchiquites differ considerably from this. Their 

 alkalies are very much lower and iron oxides higher (except as com- 

 pared with the jacupirangite), though the ratio, Na 2 : K 2 0, still remains 

 high. In the biotite-rich ouachitites, again, MgO is very high, and, 

 while total alkalies remain about the same, yet K 2 is much higher than 

 Na 2 0. 



*Pirsson : 18th Ann. Rept. Geol. Survey U. S., part iii, 1898, p. 567. 

 f Williams : Op. eit., p. 261. 



LVIir— ttin.T.. GF.or.. Soc. Am., Vof.. 11, 1899 



