314 Astronomical Society. 



great marks of disturbance. The Espichel limestone and the red 

 sandstone have been also greatly elevated at the Castle Hill at Ce- 

 zimbra, by a trap rock of which the date is uncertain. 



Although the author had innumerable opportunities of observing 

 the junction of the basalt with the beds below it, yet in no instance 

 did he observe any change in the characters of the subjacent rocks. 

 The alteration produced in the beds of shale, which alternate with 

 trap rocks near Cintra, has been already noticed. Mr. Sharpe con- 

 siders these igneous strata to have been ejected contemporaneously 

 with the deposition of the shale, and to be consequently older than 

 the great coating of basalt. The Espichel limestone, in contact 

 with the trap near Cezimbra, is also altered, being of a crystalline 

 texture to a distance of fifty feet from the igneous rock. 



Granite is found only in the neighbourhood of Cintra, forming a 

 rano-e of hills about seven miles in length and five in breadth. Their 

 greatest altitude is less than 2000 feet. The prevailing rock is a 

 true granite consisting of nearly equal proportions of quartz 

 and felspar with a little mica ; but towards the western end of the 

 chain, syenite and porphyry occur. In the central portions of the 

 hills, the granite is coarsely grained, and splits into large irregular 

 blocks; but on the flanks it is schistose, finely grained, cleaves 

 into rhombs, and might be mistaken for a sandstone. Veins of 

 large-grained granite, however, occur in the schistose variety, and 

 veins of finely-gi'ained in the coarse central masses. 



Mr. Sharpe then describes, in detail, the dislocations in the sedi- 

 mentary strata on the flanks of the granitic hills ; and he shows that 

 all the formations, from the San Pedro limestone to the Espichel, 

 have been dislocated, and thrown into highly inclined positions, but 

 the details cannot be clearly understood without the aid of sections. 

 It may however be stated, that In consequence of the red sandstone 

 restino in nearly horizontal strata against the inclined beds of the 

 lower formation, the latter was disturbed previously to the de- 

 position of the sandstone, and that consequently the irruption of 

 tlie granite of Cintra took place at a period anterior to the origin of 

 the sandstone. 



Mr. Sharpe describes also with considerable minuteness the dis- 

 turbance near Palmella. south of the Tagus ; and he infers, from the 

 relative position of the strata, that there have been, in that district, 

 considerable elevations at four distinct periods. 



The paper concludes with some observations on the earthquake 

 of 1753 ; and the author, shows, that its effects were entirely 

 confined to the tertiary strata, and were most violently felt on the 

 blue clay belonging to the Almada beds, on which the lower part 

 of the city is constructed. Not a building on the Hippurlte lime- 

 stone, or the basalt, was injured. 



ASTRONOMICAL SOCILTY. 



Jan. 11, 1839. — The following communications were read: " On 

 the Obliquity of the Ecliptic." By the Rev. Dr. Pearson. 



The object of this paper Is to give an account of a determination 

 of the annual diminution of the obliquity, by a comparison of obser- 



