Stratification. 53 with that of the fuperimpofed maffes. Sometimes, indeed, there are exceptions to this rule ; but thefe are eafily explained. It is often very difficult to determine, whether the rock we are examining be ftratified or not, and when the feams of ftratification are hid, to know the diredion of the ftrata. The following obfer- vations will be ufeful in removing fome of the dif ficulties attending luch inveftigations. 1. Strata are alrnoft always parallel with the fiaty ftrudure of the ftone. In certain porphyri- tic granites, the cryftals of felfpar appear to lie parallel with the ftrata; the latter character, however, is by no means fo decifive as the for mer. 2. Strata can only be formed by parallel feams, which have the fame direction and extent through the mountain mafs. Where parallel rents occur in different directions in the fame fpecies of rock, as in granite, fandftone, limeftone, &c. it is evi dent that they are to be confidered as acciden tal. 3. The feams of tabular diftind concretions, which are often of confiderable extent, muft not be confounded with ftrata feams, becaufe their ex tent is not fo confiderable ; and in each group of concretions the direction is different. 4. Where parallel rents have a different direc tion from the flaty. ftrudure of the ftone, they are certainly