Particle 로 or 으로 after consonant?

Question

I stumbled upon what seems to be a contradiction in my beginner's course textbook.

The book explains that the particle which indicates direction, "로", must be replaced by its variant "으로" if preceded by a consonant.

However, the examples given conflict with the previous statement:

• 바다로 - "toward the sea"
• 서울로 - "toward Seoul"
• 건물으로 - "toward the building"

Why isn't the second example "서울으로"?

Answer 1

1. "no prop consonant (받침 없음)" or "prop consonant ㄹ (받침 ㄹ)" + 로
2. the other prop consonants + 으로