130 REPORT ON THE GEOLOGY OF ESSEX CO., MASS. 



from any of the syenites previously described and was 

 named from the county of Essex, being peculiarly local 

 in its occurrence. 



EFFUSIVE VOLCANIC DYKE ROCKS. 



No. 23. Quartz-Porphyry Dykes. 



This number is placed on outcrops of this rock to dis- 

 tinguish a series of narrow quartz-porphyry dykes which 

 are of a later age than the quartz-porphyry and rhyolite 

 of the ancient volcanic series, inasmuch as these narrow 

 dykes are seen eutting the latter. 



No. 24. Arkose : Conglonierate-Granite. 



A typical granitic-breccia found at Magnolia on the 

 southwest side of Crescent beach and in Saugus Centre. 



No. 25. Diallage-Gabbro : Pyroxene Rocks. 



These are massive dykes first noticed by Dr. M. E. 

 Wadsworth. 



EFFUSIVE VOLCANIC ROCKS, YOUNGER SERIES. 



No. 26. Liparite Dyke. 



This is a dyke about seven feet wide eutting the diorite 

 and granite in Throckmorton's cove on the Marblehead 

 side of Forest river. Thin sections show that the ground 

 mass of this rock is composed of a f elting of sanidine crys- 

 tals, enclosing numerous long porphyritic crystals of 

 sanidine ; quite large crystals of quartz, surrounded by a 

 f ringe of spherulites, and having inclusions of augite and 

 hornblende crystals ; blebs of chalcedony, surrounded by 

 a ferrugineous feathery mineral, and the whole thickly 

 covered with spherulites. 



CRYSTALLINE ROCKS OF CLASTIC ORIGIN. 



No. 27. Red-Slate : Jaspelite. 



This rock oecurs in Saugus Centre, Lynn and Nahant. 



